INTRASPECIFIC VARIATION IN THE LIGHT/DARK MODULATION OF RIBULOSE 1,5-BISPHOSPHATE CARBOXYLASE-OXYGENASE ACTIVITY IN SOYBEAN

A comparative study was made of the inhibition of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) amongst six cultivars of Glycine max L. Merr., associated with synthesis of 2-carboxyarabinitol 1-phosphate (CA1P) during darkness. Significantly lower mean values of dark inhibition of Rubisco were observed in soybean cv. Davis than in cvs Bragg, Cobb, Hardee, Gordon, and Kirby. The CA1P synthesis/degradation cycle during dark/light transitions remained operational in cv. Bragg plants grown at low irradiance (40 mu mol photons m(-2) s(-1)). However, CA1P synthesis and degradation rates were slower in the dark (t(0.5) = 240 versus 25 min), and light (t(0.5) = 20 versus 3.8 min) respectively, as compared to plants grown at higher irradiance (550 mu mol photons m(-2) s(-1)). In addition, the activation state of Rubisco in low-light-grown plants showed only a small decline after a transition to darkness. We conclude that (a) cultivar-dependent variation occurs amongst soybeans with respect to CA1P regulation of Rubisco, and (b) soybeans acclimated to low irradiance may depend more on CA1P synthesis/degradation to regulate Rubisco, and less on changes in the enzyme activation state.